Collapsible “Buckliball” turns failure into functionality

Taking inspiration from a toy, a team of researchers at MIT have developed a new engineering structure that is mechanically unstable, yet collapses in a way that is predictable and reversible. The structure, formed out of a single piece of rubber-like material, is fabricated so that it collapses in harmony to form a smaller structure that can then be expanded into the original shape. This structure opens up new potentials in everything from architecture to micro-medical applications.

When we think of structures, we tend to think of them as things that don’t fall down. If you had to come up with one common criterion for bridges, buildings, houses, stadiums, sheds and dog houses, it’s that once built, they should tend to stay upright and not come crashing down around people’s ears. If they do so, that is generally regarded as a failure, so engineers put a great deal of effort into keeping that from happening.

Nonetheless, a team at the Massachusetts Institute of Technology led by Katia Bertoldi, an assistant professor in applied mechanics at Harvard, and Pedro Reis, the Esther and Harold E. Edgerton Assistant Professor of Civil and Environmental Engineering and Mechanical Engineering at MIT, were looking for a engineering structure that was intended to collapse. Its function was to fall down.

No, this wasn’t meant as some sort of practical joke. The idea was to have the structure collapse in a controllable, predictable fashion that could then be reversed, with the structure going back to the way it was. Think of it as being a bit like those old-fashioned cups made out of nesting rings that you could pull out to form a drinking vessel, or paper origami that folds down or pops up when you pull on it the right way. The origami analogy is very close because, according to the MIT team, what they were looking for was a “buckling-induced origami” they call “buckligami.”

The idea was simple in theory, but frustrating in practice. It didn’t just need to be a shape that could be folded or unfolded. It had to be a shape that would collapse completely from one form to another. The answer kept eluding them until one of the team noticed a toy ball made up of t-shapes, pivots and linkages that, when pressed, did exactly what the MIT team was looking for - it fell in on itself in a state of complete collapse until it formed a smaller ball. Pulling on it made it expand, evenly reforming itself into the original, larger ball. It was the model they were looking for.

From toy to breakthrough

In a way, this isn’t too surprising. Toys are big business and a lot of cutting-edge or, at least, very clever science and technology goes into their development. Many of modern history’s most popular toys started out a laboratory curiosities or were the product of incredible research and development efforts. Sometimes, toys return the favor.

Go into any modern development lab and there’s a good chance that there will be some equipment cobbled together out of parts from a toy construction set, or using bits cannibalized from some plaything that can't be found by its seven-year old owner. In this case, what this particular toy ball contributed was the very idea behind its construction. The bits of the ball were put together in such a way that all the pieces that held the ball together in its spherical shape could be made, with a slight pressure, to lose all structural integrity simultaneously and fold in on themselves.

It’s like a building where every single girder fails at the same time or, to use a more realistic example, whenever I try to put up a tent. The only difference is that here you end up with a pile of rubble or a heap of nylon and poles while the toy ball collapses into another ball.

This example allowed the MIT team to create the simplest three-dimensional structure that could take advantage of mechanical instability to reversibly collapse. Using 3D printing techniques, they made a hollow sphere out of a rubber-like material. It had no moving parts. Instead, it was fashioned with 24 carefully spaced dimples. They called this a “buckliball” because of its resemblance to the buckyball carbon nanostructures, plus it’s a play on the phrase “bucklely ball." This is why scientists don’t write advertising copy.

These buckliballs hold their shape quite nicely when inflated, but take the air out and they fold in on themselves quite dramatically. The thin ligaments between the dimples collapse and all the bits (or the structure) start to move in a remarkably orderly fashion. They just sort of slide past one another in a rubbery dance. Some bits go clockwise, others go anticlockwise and it all folds into a nice little “rhombicuboctahedron" – that’s an irregular geometric solid with eight triangular and 18 square faces, if your Euclid is a bit rusty. It’s also the first morphable structure to incorporate buckling as a engineering element. That’s pretty impressive for a bit of plastic without any moving parts.

Applications

“In civil engineering, buckling is commonly associated with failure that must be avoided," said Dr Reis. "For example, one typically wants to calculate the buckling criterion for columns and apply an additional safety factor, to ensure that a building stands. We are trying to change this paradigm by turning failure into functionality in soft mechanical structures. For us, the buckliball is the first such object, but there will be many others.” What that means is that a material that can collapse predictably on order and return to its original shape could be very useful.

One area of application could be in engineering. Buildings could be made with collapsible roofs and walls. Remodelling could be a matter of giving the wall a good shove and watching it tuck itself away. You could have real folding chairs - maybe one where the couch folds into a love seat. Robots could be built with buckliballs where the hinges go to provide a robot arm with more strength for less weight. You could even make robot skin that takes the place of a motor, making it even lighter and more compact.

Then there are medical applications. Something like a buckliball already exists in nature. Some viruses inject their DNA into host cells by means of buckliball-like structures that they use as a sort of nano squeezy bottle. Reis believes that by imitating this, doctors could design drug delivery systems with an incredible degree of precision. The implications in treating cancer, for example, by applying drugs just to the cancerous cells in chemotherapy could save a lot of suffering.

Ironically, one other application that the MIT team see is in toys. They say that materials derived from the buckliball could be used to design transformers that make current morphing toys look unbelievably lame by comparison.